Exhaust gas after-treatment method

ABSTRACT

An exhaust gas after-treatment method includes determining a regeneration condition of a particulate matter filter, eliminating trapped particulate matter to a predetermined amount by raising a temperature of exhaust gas to a first value so as to regenerate the particulate matter filter. An exhaust gas flows into a nitrogen oxide purifier is controlled in a lean condition (air/fuel ratio &gt;14.5) while the particulate matter filter is being regenerated sulfur components are eliminated from the nitrogen oxide purifier by controlling exhaust gas that flows into the nitrogen oxide purifier after the lean condition is performed. Subsequently, the lean condition and a rich condition (air/fuel ratio &lt;14.5), and then completes the regeneration of the particulate matter filter is completed or the elimination of the sulfur component from the nitrogen oxide purifier is completed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2012-0143086 filed in the Korean IntellectualProperty Office on Dec. 10, 2012, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an exhaust gas after-treatment method,and more particularly, to an exhaust gas after-treatment methodcontrolling a temperature of exhaust gas to eliminate sulfur from anitrogen oxide purifier and particulate matter from a particulate matterfilter.

BACKGROUND

Generally, a particulate matter filter filters particulate matter (PM)included in exhaust gas. Further, the filtered particulate matter isburned at high temperature to be eliminated. The elimination of PM iscalled “regeneration”.

A nitrogen oxide purifier (LNT: lean NOx trap) absorbs nitrogen oxideincluded in the exhaust gas where the nitrogen oxide is reduced by areaction with a reducing agent under predetermined conditions.

In a case in which the nitrogen oxide purifier is disposed at adownstream side of an exhaust manifold, and the particulate matterfilter at a downstream side of the nitrogen oxide purifier, sulfurcomponents move with the exhaust gas to be continuously absorbed in thenitrogen oxide purifier.

The sulfur compound absorbed in the nitrogen oxide purifier deterioratesthe nitrogen oxide purification performance, and therefore, the sulfurcomponent has to be cyclically eliminated.

Further, a high temperature condition is necessary to eliminate thesulfur component while the particulate matter filter is beingregenerated. A process for eliminating the sulfur component from thenitrogen oxide purifier during the regeneration of the particulatematter filter has been developed. However, when the sulfur componentsare eliminated during the regeneration of the particulate matter filter,the nitrogen oxide purifier can be damaged by an excessive hightemperature, thus deteriorating the purification performance.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the disclosure, andtherefore may contain information that does not form the prior artalready known in this country to a person of ordinary skill in the art.

SUMMARY

The present disclosure has been made in an effort to provide an exhaustgas after-treatment method having advantages of preventing a nitrogenoxide purifier in a sulfur elimination process from overheating during aregeneration of a particulate matter filter.

An exhaust gas after-treatment method according to an exemplaryembodiment of the present disclosure comprises determining aregeneration condition of a particulate matter filter, eliminatingtrapped particulate matter up to a predetermined amount by raising atemperature of exhaust gas up to a first value so as to regenerate theparticulate matter filter. Exhaust gas that flows into a nitrogen oxidepurifier is controlled in a lean condition (air/fuel ratio >14.5) whilethe particulate matter filter is being regenerated. Sulfur componentsare eliminated from the nitrogen oxide purifier by controlling exhaustgas that flows into the nitrogen oxide purifier after the lean conditionis performed. The lean condition and a rich condition (air/fuel ratio<14.5) are repeated. The regeneration of the particulate matter filteror the elimination of the sulfur components from the nitrogen oxidepurifier is completed.

The nitrogen oxide purifier may be disposed at a downstream side of theparticulate matter filter or disposed at a downstream side of theexhaust manifold.

The first value may range from 600 to 700 degrees Celsius. Thetemperature of exhaust gas may be raised to the first value in multiplesteps to protect the particulate matter filter. The temperature of theexhaust gas flowing into the nitrogen oxide purifier may be controlledto be lower than 700 degrees Celsius such that the particulate mattertrapped in the particulate matter filter is eliminated in the leancondition or may be controlled to be between a sulfur eliminationminimum temperature and a sulfur elimination maximum temperature in theeliminating of the sulfur components from the nitrogen oxide purifier.

The regeneration condition of the particulate matter filter may bedetermined by an amount of the particulate matter trapped in theparticulate matter filter, travel distance, a travel time, or afront/rear side pressure difference of the particulate matter filter.

The temperature of the exhaust gas flowing into the particulate matterfilter or the nitrogen oxide purifier may be controlled by a maininjector injecting fuel into a combustion chamber, an intake aircontroller, or a post-injection injector.

The completion of the regeneration of the particulate matter filter maybe determined by the particulate matter trapped in the particulatematter filter, front/rear pressure difference, or regeneration time.

The completion of the sulfur elimination of the nitrogen oxide purifiermay be determined by a sulfur amount stored in the nitrogen oxidedevice, a sulfur elimination time, or a sum of the rich conditionduration time in the sulfur elimination process.

The exhaust gas may be controlled to be in a lean condition when thesulfur elimination of the nitrogen oxide purifier is completed, and theregeneration of the particulate matter filter device is not completed.

As described above, in an exhaust gas after-treatment method, afterexhaust gas is controlled to be in a lean condition, the exhaust gas iscontrolled to be in a rich condition so as to eliminate sulfurcomponents from the nitrogen oxide purifier during the regeneration ofthe particulate matter filter to prevent overheating of the nitrogenoxide purifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exhaust gas after-treatment systemaccording to an exemplary embodiment of the present disclosure.

FIG. 2 is a flowchart showing an exhaust gas after-treatment methodaccording to an exemplary embodiment of the present disclosure.

FIG. 3 is a graph showing temperature of a nitrogen oxide purifier in anexhaust gas after-treatment method according to an exemplary embodimentof the present disclosure.

DETAILED DESCRIPTION

An exemplary embodiment of the present disclosure will hereinafter bedescribed in detail with reference to the accompanying drawings.

Referring to FIG. 1, an exhaust gas after-treatment system illustratesan exemplary embodiment for understanding of the present disclosure.Further, the present disclosure is not limited thereto and can beapplied to various engine systems.

An exhaust gas after-treatment system includes an intake line 100, athrottle valve 105, an intake manifold 110, a combustion chamber 120, amain injector 125, an exhaust manifold 130, a post-injection injector132, a temperature sensor 135, oxygen sensors 137 and 162, an exhaustline 160, a nitrogen oxide purifier 140, a particulate matter filter150, and a differential pressure sensor 155. The main injector 125,which can perform the function of the post-injection injector withoutthe post-injection injector, injects fuel into the combustion chamber120, and the post-injection injector 132 injects fuel into the exhaustline 160. The main injector 125 generates engine torque through fuelcombustion. Further, the main injector 125 and the post-injectioninjector 132 control the condition of exhaust gas passing through theexhaust line 160 to a lean or rich state and controls the temperature ofthe exhaust gas.

The oxygen sensors 137 and 162 detect oxygen concentration of theexhaust gas to transmit detected signals to a controller 170. Thecontroller 170 determines whether the condition of the exhaust gas islean or rich based on the received signals.

The throttle valve 105 controls intake air amount, the temperaturesensor 135 detects the temperature of the exhaust gas passing throughthe exhaust line 160, and the differential pressure sensor 155 detectspressure difference between upstream side and downstream side of theparticulate matter filter.

The controller 170 determines whether the regeneration condition foreliminating soot trapped in the particulate matter filter 150 issatisfied or not based on the differential pressure signal, traveldistance, or travel time.

When the regeneration condition for the particulate matter filter 150 issatisfied, the temperature of the exhaust gas is raised to about 650degrees Celsius to regenerate the particulate matter filter 150.

Further, the controller 170 performs a desulfation mode of the nitrogenoxide purifier 140 when the particulate matter trapped in theparticulate matter filter 150 is reduced to a predetermined level.

When the desulfation mode begins, the condition of the exhaust gasflowing into the nitrogen oxide purifier 140 is controlled to the leanstate such that the temperature of the exhaust gas flowing into thenitrogen oxide purifier 140 is securely sustained. Also, the exhaust gasflowing into the nitrogen oxide purifier 140 is controlled to be in therich state, such that sulfur compounds are eliminated.

The particulate matter filter 150 is continuously regenerated while theexhaust gas is repeatedly cycled from the lean to the rich state and therich to the lean state.

After the desulfation of the nitrogen oxide purifier 140 is completed,the exhaust gas is controlled to be in the lean state to complete theregeneration of the particulate matter filter 150.

FIG. 2 is a flowchart showing an exhaust gas after-treatment methodaccording to an exemplary embodiment of the present disclosure.

Referring to FIG. 2, step S200 determines the regeneration condition ofthe particulate matter filter 150. The regeneration condition issatisfied when the soot amount trapped in the particulate matter filter150, the travel distance, the travel time, or the pressure differencebetween an upstream side and a downstream side of the particulate matterfilter 150 are within predetermined conditions.

The temperature of the exhaust gas flowing through the exhaust line 160is raised to a temperature T1 (about 650 degrees Celsius) in step S210.In this step, the temperature of the exhaust gas may be raised inmultiple steps by the main injector 125, the post-injection injector132, or an intake air controller.

The particulate matter trapped in the particulate matter filter 150 iseliminated to a predetermined level M1 in step S220. In this step, thepredetermined level M1 may be a value at which the particulate matterfilter cannot be damaged by DTI (Drop To Idle) or DTO (Drop To Overrun).

In a case in which the sulfur compound is eliminated from the nitrogenoxide purifier 140 after the particulate matter filter 150 is completelyregenerated, the time that the exhaust gas temperature is sustained at ahigh temperature longer, and the particulate matter filter 150 and thenitrogen oxide purifier 140 may be degraded by the high temperature.After a predetermined amount of particulate matter trapped in theparticulate matter filter 150 is eliminated, the desulfation mode of thenitrogen oxide purifier 140 reduces fuel consumption and prevents thedegradation of the particulate matter filter 150 and the nitrogen oxidepurifier 140.

In step S230, the condition of the exhaust gas flowing into the nitrogenoxide purifier 140 is controlled to be in a lean state. Accordingly, thetemperature of the nitrogen oxide purifier 140 is securely sustained tobe less than 700 Celsius degrees.

After the exhaust gas in the nitrogen oxide purifier 140 is in the leanstate, the exhaust gas is transformed to a rich state again. In thisstep, the temperature of the nitrogen oxide purifier 140 is controlledto be between the desulfation minimum temperature and desulfation highmaximum temperature, thereby eliminating the sulfur compound absorbed inthe nitrogen oxide purifier.

The condition of the exhaust gas alternately repeats the lean state andthe rich state in step S240. Afterwards regenerating the particulatematter filter 150, the desulfation of the nitrogen oxide purifier 140 iscompleted in step S250.

The exhaust gas regenerates the particulate matter filter 150 in a leanstate after the desulfation of the nitrogen oxide purifier 140 iscompleted.

The regeneration completion of the particulate matter filter 150 may bedetermined when amount of trapped particulate matter is less than apredetermined level, the regeneration time passes a predetermined value,or the pressure difference is less than a predetermined value.

The desulfation condition of the nitrogen oxide purifier 140 is when theamount of sulfur compound thereof is less than a predetermined value,the desulfation time is longer than a predetermined value, or the sum ofthe rich condition sustaining time is longer than a predetermined value.

The time that the exhaust gas is being sustained in a lean state and theexhaust gas in a rich state can be predetermined. The time can be variedby the exhaust gas temperature in the particulate matter filter 150 orthe nitrogen oxide purifier 140.

FIG. 3 is a graph showing temperature of a nitrogen oxide purifier in anexhaust gas after-treatment method according to an exemplary embodimentof the present disclosure.

Referring to FIG. 3, a horizontal axis denotes time and a vertical axisdenotes temperature of nitrogen oxide purifier. The temperature of thenitrogen oxide purifier 140 or the particulate matter filter 150 can becalculated with the temperature of the exhaust gas where the exhaust gastemperature may be the temperature of the nitrogen oxide purifier 140 orthe particulate matter filter 150.

As shown in the drawings, the temperature of the exhaust gas is raisedfrom about 600 to 700 degrees Celsius so as to regenerate theparticulate matter filter 150.

The desulfation of the nitrogen oxide purifier 140 is performed duringthe regeneration of the particulate matter filter 150. Here, a firsttemperature line in which the exhaust gas condition is transformed froma lean to a rich state and a second temperature line in which theexhaust gas condition is transformed from a rich to a lean state havedifferent characteristics from each other.

As shown in the first temperature line, the temperature of the nitrogenoxide purifier 140 descends to be stable. And, the temperature thereofis sustained to be from 650 to 700 degrees Celsius again. In the secondtemperature line, there is a danger that the temperature of the nitrogenoxide purifier 140 can be raised to exceed 700 degrees Celsius, and thiscondition can damage the nitrogen oxide purifier 140 or can deteriorateexhaust gas purification performance.

As described above, only predetermined amount of the particulate mattertrapped by the particulate matter filter 150 is eliminated, and thedesulfation mode of the nitrogen oxide purifier 140 is performed toreduce the fuel consumption and to prevent the degradation of theparticulate matter filter 150 and the nitrogen oxide purifier 140.

Further, while the particulate matter filter 150 is being regenerated,after the exhaust gas is in a lean state so as to eliminate sulfur fromthe nitrogen oxide purifier 140, the exhaust gas is controlled to be ina rich state to securely control the temperature of the nitrogen oxidepurifier 140 and prevent the degradation thereof.

While the disclosure has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the inventive concept is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. An exhaust gas after-treatment method, comprising: determining a regeneration condition of a particulate matter filter; eliminating a trapped particulate matter up to a predetermined amount by raising a temperature of an exhaust gas to a first predetermined value so as to regenerate the particulate matter filter; controlling an exhaust gas that flows into a nitrogen oxide purifier in a lean condition while the particulate matter filter is being regenerated; eliminating sulfur components from the nitrogen oxide purifier by controlling exhaust gas that flows into the nitrogen oxide purifier after the lean condition is performed; and repeating the lean condition and a rich condition, and completing the regeneration of the particulate matter filter, or completing the elimination of the sulfur components from the nitrogen oxide purifier.
 2. The exhaust gas after-treatment method of claim 1, wherein the first value ranges from 600 to 700 degrees Celsius.
 3. The exhaust gas after-treatment method of claim 1, wherein a temperature of exhaust gas is raised to the first value in multiple steps to protect the particulate matter filter.
 4. The exhaust gas after-treatment method of claim 1, wherein the nitrogen oxide purifier is disposed at a downstream side of the exhaust manifold.
 5. The exhaust gas after-treatment method of claim 1, wherein a temperature of the exhaust gas flowing into the nitrogen oxide purifier is controlled to be lower than 700 degrees Celsius such that the particulate matter trapped in the particulate matter filter is eliminated in the lean condition.
 6. The exhaust gas after-treatment method of claim 1, wherein the temperature of the exhaust gas flowing into the nitrogen oxide purifier is controlled to be between a sulfur elimination minimum temperature and a sulfur elimination maximum temperature in the eliminating of the sulfur components from the nitrogen oxide purifier.
 7. The exhaust gas after-treatment method of claim 1, wherein the regeneration condition of the particulate matter filter is determined by an amount of the particulate matter trapped in the particulate matter filter, a travel distance, a travel time, or a front/rear side pressure difference of the particulate matter filter.
 8. The exhaust gas after-treatment method of claim 1, wherein the temperature of the exhaust gas flowing into the particulate matter filter or the nitrogen oxide purifier is controlled by a main injector injecting fuel into a combustion chamber, an intake air control device, or a post-injection injector.
 9. The exhaust gas after-treatment method of claim 1, wherein a completion of the regeneration of the particulate matter filter is determined by an amount of the particulate matter trapped in the particulate matter filter, a front/rear pressure difference, or a regeneration time.
 10. The exhaust gas after-treatment method of claim 1, wherein a completion of the sulfur elimination of the nitrogen oxide purifier is determined by a sulfur amount that is formed in the nitrogen oxide device, a sulfur elimination time, or a sum of the rich condition duration time in the sulfur elimination process.
 11. The exhaust gas after-treatment method of claim 1, wherein the exhaust gas is controlled to be in a lean condition when the sulfur elimination of the nitrogen oxide purifier is completed and the regeneration of the particulate matter filter is not completed. 